This follow-up study to ACTG 175 will determine the influence of virus load, CD4 cell counts, biologic phenotype, and the presence of symptomatic HIV disease on the response to a new therapeutic regimen, on the kinetics of viral replication, and on the rate of emergence of 3TC-associated resistance mutation at codon 184. It will compare the efficacy of continuing ZDV/ddI or ZDV/ddC versus adding 3TC or switching to ZDV/3TC. (ACTG 303)